Tuesday, September 25, 2012

New paper shows dimming of sunshine during the 1970's ice age scare, and brightening since the 1980's

A paper published today in Atmospheric Chemistry and Physics examined trends in surface solar radiation [SSR] over Switzerland from 1885-2010. The authors find surface solar radiation increased during the 1930-1940's and then declined until the ice age scare of the 1970's, followed by an increase over the period 1980-2010. These variations are inversely correlated to cloud cover, and correlate with global temperature variations much more than the steady rise in CO2 levels over these periods. The surface solar radiation measurements show variations of about 10 W/m2, around 10 times the alleged surface forcing from a doubling of CO2 levels.According to the authors,

"A dimming (brightening) is clearly visible in all-sky SSR during the 1950s–1970s (1980s–2000s) subperiod, in line with previous studies that used a lower density of stations. Equally, there is a brief early brightening period in Switzerland restricted to the 1940s, with a lack of signiﬁcant decadal variations in the preceding period"

The paper corroborates others which have shown from various sites that surface solar radiation has markedly increased since the 1980's, primarily due to a decrease in cloud cover.Related:

A. Sanchez-Lorenzo and M. WildInstitute for Atmospheric and Climate Science, ETH Zurich, Zurich, SwitzerlandAbstract. Our knowledge on trends in surface solar radiation (SSR) involves uncertainties due to the scarcity of long-term time series of SSR, especially with records before the second half of the 20th century. Here we study the trends of all-sky SSR from 1885 to 2010 in Switzerland, which have been estimated using a homogenous dataset of sunshine duration series. This variable is shown to be a useful proxy data of all-sky SSR, which can help to solve some of the current open issues in the dimming/brightening phenomenon. All-sky SSR has been fairly stable with little variations in the first half of the 20th century, unlike the second half of the 20th century that is characterized also in Switzerland by a dimming from the 1950s to the 1980s and a subsequent brightening. Cloud cover changes seem to explain the major part of the decadal variability observed in all-sky SSR, at least from 1885 to the 1970s; at this point, a discrepancy in the sign of the trend is visible in the all-sky SSR and cloud cover series from the 1970s to the present. Finally, an attempt to estimate SSR series for clear-sky conditions, based also on sunshine duration records since the 1930s, has been made for the first time. The mean clear-sky SSR series shows no relevant changes between the 1930s to the 1950s, then a decrease, smaller than the observed in the all-sky SSR, from the 1960s to 1970s, and ends with a strong increase from the 1980s up to the present. During the three decades from 1981 to 2010 the estimated clear-sky SSR trends reported in this study are in line with previous findings over Switzerland based on direct radiative flux measurements. Moreover, the signal of the El Chichón and Pinatubo volcanic eruption visible in the estimated clear-sky SSR records further demonstrates the potential to infer aerosol-induced radiation changes from sunshine duration observations.